Contoured spot-welded lap joint and method



July 3, 1962 c. N. ANDERSON CONTOURED SPOT-WELDED LAP JOINT AND METHOD Filed NOV. 28, 1958 x T \Q N l.

INVENTOR. 6 44421455 A/. ANDERSON W H MM RF 2%, vywwvw W w W 3,042,159 CONTGURED SPOT-WELDED LAP JOINT AND METHOD Charles N. Anderson, Kent, Wash, assignor to Boeing gnplane Company, Seattle, Wash., a corporation of e aware Filed Nov. '28, 1958, Ser. No. 776,816 Claims. (Cl. 189--36) It has been observed that the normally constructed spot-welded lap joints in stressedskins or the like, such as those used in aircraft construction, and which in use are subjected to repeated tensional stresses across the joint, tend to fatigue and fail after a reasonably short period of time. When the joints are made in the conventional manner, the flat edge of one sheet member is lapped over the like edge of another sheet member, and the two are joined by rows (usually five) of spotweld nuggets along thezone of overlap. Even though there be five such rows of spotweld nuggets, and even though these nuggets be spaced rather closely apart, the fatigue life of the joint as a whole has been found unduly short. The reference to spotweld nuggets applies also to and is intended to include a riveted interconnection.

Each joint so formed has areas in each skin, lying between rows of spotwelds, which areas lie respectively at increasing distances from its margin. Such an area closest to the margin of a first skin overlaps an area in the second skin that is farthest from the latters margin but closest to the point of application of a tensioning force; the next inner area in the first skin overlaps an area nearer the margin of the second skin, and so on until nearest the margin of the second skin the overlapping area of the first skin is farthest from the latters margin, but is itself nearest the point of application to the first skin of the tensioning force. Critical examination of conventional joint specimens under test conditions reveals that these overlapping areas at differing distances from their respective margins, of identical width when unstressed, stretch by different amounts from each other under given tension. The area nearest a margin, and so farthest from the point of application of stress, will stretch less than the area which it overlaps, and which is farthest from its margin but nearest the point of application of the force. This produces unequal lateral elongation in the respective overlapped areas, and this sets up forces which act locally upon each spotweld nugget or other fastening means (rivets, etc.) tending to distort them, as will be explained further with reference to FIGURE 4 of the accompanying drawings.

According to the present invention the skins in the zone of overlap are complementally contoured, usually before joining them, in such manner that after they are joined, as by rows of spotwelds, the stretching which will tend to flatten the contours, and so would otherwise produce the unequal lateral elongation will serve merely to return the overlapped areas towards equal dimensions, the contouring having precompensated for such unequal elongation. In this manner, as tests have shown, a joint constructed according to the present invention has a fatigue life at least many times as long as that of conventional joints.

The present invention comprises a lapped splice joint, preferably spotwelded, between two skin elements or the like which in use lie more or less in a common plane, of the sort briefly described above. There is described also herein a process of forming such a joint. The invention is shown in a representative form in the accompanying drawings, and will be more fully explained hereinafter, and defined in the claims.

FIGURE 1 is a sectional View transversely of a lapped ite states Patent "ice splice joint according to this invention, and FIGURE 2 is an isometric face view of the same.

FIGURE 3 is an enlarged sectional view illustrating a marginal portion of one skin element and the overlapped portion of the other, with two rows of spotweld nuggets and the lapped areas therebetween, illustrating the forces active in a joint according to the present invention.

FIGURE 4 is a view similar to FIGURE 3, but illustrating, exaggeratedly, the play of forces in a conventional lap splice joint.

Herein when reference is made to an overlapping portion or an underlapping portion of the contoured margins of the skin elements, the reference does not refer to orientation with respect to the horizon, but rather to the concavity or convexity of the marginal portions. A portion the faying surface whereof is concave will be considered to overlap the complemental convex faying surface of the other portion. The second portion will be considered to underlap the first portion.

In principle, the present invention involves the complemental contouring of the zones along the edges of two skin elements, where they are to lap, or are lapped orienting the contoured portions correctly in registry, and joining them by rows of fastening elements, normally spotweld nuggets, although the same principles will apply if rivets or other such fastening elements are used, and such equivalents are intended to be included where spotweld nuggets are referred to. The contouring is of a special nature, being in effect a shallow reverse curve in crosssection, with the point of reversal or common point of tangency approximately midway between the marginal edges of the overlapped portions. The correct registry is also important, that is to say, a given sheet area should overlap, not underlap, the complemental area of the other 5 sheet. Two rows of spotwelds extend adjacent the respective marginal edges of the skin elements, and one or more rows extend along an intermediate line or lines. Normally a single intermediate row will suffice, located along the neutral line where the contour curvature reverses. The contours are so related that the contacting or faying surfaces of the skin elements are contoured to identical curvatures; thereby the convexly curved overlapping areas (considering their disposition when the joint is completed), nearest the marginal edge of each skin element, is stretched slightly initially about a neutral plane defined by its eventually concave surface, whereas the immediately underlapped concave area may be slightly compressed initially, also about a neutral plane at the faying, eventually convex surface thereof. Now when these mating areas are joined adjacent the margins of the skins, as by two rows of spotweld nuggets, and by an intermediate row or rows, such as a third row along the line of the common points of tangency, and are subsequently tensioned, each precornpressed underlapping concave area tends to extend or stretch, or the corresponding pretensioned overlapping convex areas tend to contract, either or both, the net result tending to flatten the contouring of the joint as a whole, to the initial planar form of the sheets, and there is no such distortional force produced as before, acting locally upon the spotweld nuggets to separate the skin elements from the individual nuggets. The joint so formed has a fatigue life a number of times greater than such joints conventionally formed. Whether or not the reasons for such long life are as given, the result is found to follow the production of the joint in the manner described. It has been found that a threerow joint so produced will have a fatigue life much longer than a conventional flat five-row joint, so that a further advantage of this invention lies in lessening the cost of such joints.

The two elements to be jointed are represented at 1 and at 2. Beyond the joint they lie generally in a common plane defined approximately in FIGURE 1 by the broken stub ends of the skin elements 1 and 2. The marginal portion of each skin element laps such portion of the other, and lies in contact therewith throughout the lapping area. Each marginal lapping portion is very shallowly but reversely contoured. Each departs from the original plane of its sheet along a line of tangency at radius A, in FIGURE 1, for skin element 1, and curves to a line B on a long radius R1 about a neutral plane defined by the ultimately faying surface 11; at the line B its curvature reverses, the intersection of this radius and the faying surfaces being a point of tangency common to the two curves, and defining a line of neutral stress, and extends in a smooth reverse curve struck from the center of radius R2, again about the neutral plane efined by surface 11, to the marginal edge of skin element 1. The sheet 2 is complementally contoured. It is to be noted that the radii R and R terminate at the contiguous skin surfaces 11 and 12, or to put it differently, the stress in the skin elements is about a neutral plane defined by the faying surfaces 11 and 21. As a result (referring now to FIGURE 3) that exposed skin surface which in the finished joint is convex is stretched, so that any two laterally spaced points such as M and N, that lie elsewhere than in its concave surface 11, and equidistant from a given skin surface, are spaced some what farther apart than they were before contouring. Conversely, the area of skin element 2 that immediately underlaps the same area of skin element 1 has its ultimately exposed surface concavely contoured, and any two laterally spaced points P and Q that lie elsewhere than in the surface 21, and which were originally spaced apart identically with the original spacing of points M and N, are spaced slightly closer together than they were before contouring. The ideal condition is that points M and P lie in line B and points N and Q in line A, when the contoured joint is unstressed. Exactly the same result occurs at the opposite side of the common point of tangency, at B, althrough reversed in effect as between skin elements 1 and 2. Underlapping concave skin element 1 is there compressed laterally, and overlapping convex skin element 2 is stretched.

The radii R1 and R2 are preferably the same. Their actual value will depend on various circumstances, including the thickness of the skin elements, the aerodynamic tolerance to departure from precise smoothness, etc. It appears that a radius R1 and R2 of sixteen inches, in aluminum alloy sheets .040" thick, of type 2024-T3 al clad, is optimum, but this is given by way of example and not of limitation. Such a radius appears to cause no appreciable increase in drag in a given aerodynamic design, and is adequate to increase the life of the joint.

With the areas of overlap thus contoured, they are overlaid in contiguity throughout, sheet 1 oriented as shown relative to sheet 2, and not the reverse thereof, and while unstressed (other than by the contouring) are joined along several rows, by fastening elements such as the spotweld nuggets 31, 32, 33. Assuming the use of two marginal rows and a single intermediate row, the row 33 is located along the line of common tangency, or of neutral stress, the row 31 adjacent the marginal edge of skin element 1, and the row 32 adjacent the marginal edge of skin element 2. Additional rows could be added, for example, two intermediate rows, spaced apart, might replace the single row 33, but have been found unnecessary, and might even prove to be detrimental under some conditions.

Now when the skin elements so contoured, oriented, and joined are tensioned across the joint, there results a tendency for the contoured marginal overlapped and underlapped portions of the joint to flatten out The tendency is concentrated along the median plane of the joint as an entity, which median plane is approximately defined by the faying surfaces 11 and 21. Whatever was displaced to either side of this plane tends to approach it. All the convex curvature tends to compress, in the direction of stress, and all the concave curvature tends to elongate in that direction. Specifically, points M and N tend to reapproach, and P and Q again separate. Whereas by the contouring points M and N were spaced by the distance D1, slightly greater than their spacing in the flat, and greater than the distance D2 between points P and Q, which is less than their spacing in the flat, tensioning tends to restore the distances D1 and D2 to their original equality. The ultimate result is that there is no tendency to pull spotweld nuggets 31 toward nugget 33, nor to rotate nugget 31 with relation to the skin elements which it oins.

In the conventional joint of like nature, shown in FIG URE 4, tension in the skin elements 1a and 2a across the joint tends to stretch the skin 2a between joints P and Q, nearest the application of the force, to a greater extent than the skin in is stretched between corresponding points M, N, which are farthest from the application of force to skin 1a; compare distances D1 and D2, which are equal when the joint is unstressed. This produces a rotational stress acting upon the spotweld nugget 31a, represented by the arrow C, and failure is initiated there. It was found that the marginal edge of skin 1a actually curled up, as shown, although somewhat exaggeratedly. Similar results occur at the opposite marginal edge, but here stretching between P and Q" is less than between N, N", also P'Q P'Q", and M'N M'N. Extra rows of spotwelds may slow the failure, but once the outer row of spotwelds has failed appreciably the next row must assume more than its intended portion of the load, and quickly fails.

With a joint formed according to this invention the rotational effect on the spotweld nuggets does not occur, or is greatly decreased. Tests have shown a fatigue life in a joint made according to this invention that is many times the life of the conventional joint, when subjected to design loads. Specimens under test, in materials of the type specifically mentioned earlier, have been found to have an average cycle life close to one million cycles.

'It has been assumed that the overlapped marginal portions of each skin element are contoured prior to joining them, and this is the preferred procedure. It is diflicult under some conditions to maintain contours which mate perfectly throughout their entire areas. If such difficulty arises, it may prove feasible to first tack or join the overlapping marginal areas in the flat, and then to contour them as described above, keeping the neutral line at the mating surfaces of the joint. This will still displace the overlapped areas and relocate points M and N, P and Q as before, and appears likely to produce approximately the same results and advantages. Also, the contouring may be produced during the welding operation. The joint structure, in any case, is that described above, and has the specified advantages, hence the claims to the joint are not to be understood as restricted to any single method of production.

I claim as my invention:

1. A structural joint adapted to withstand tensional stresses, comprising a pair of sheet-like members having lapped surfaces in faying contact with one another to form an interface extending over substantially the entire area of lap between the surfaces, and means along the interface interconnecting the surfaces to transmit said stresses across the joint, said surfaces being so complementally contoured that said interface is curvilinear when the members are in the unstressed state and rotational forces occurring across said member during stressing are counteracted to prevent separation of the surfaces.

2. A structural joint adapted to withstand tensional stresses, comprising a pair of sheet-like members having lapped surfaces in faying contact with one another to form an interface extending over substantially the entire area of lap between the surfaces, and means along the interface interconnecting the surfaces to transmit said stresses across the joint, said surfaces being so complementally contoured that said interface is generally sinusoidal when the members are in the unstressed state and the neutral axis of contour in each of said members is disposed adjacent the interface.

3. A structural joint as set forth in claim 2 wherein said interconnecting means includes a row of spotweld nuggets.

4. A structural joint as set forth in claim 2 further characterized in that said interconnecting means includes three rows of spaced fastening means, two of which lie adjacent the margins of said members and one of which lies intermediate said first tWo rows and being positioned along the line of the reversal of curvature between said members.

5. A structural joint as set forth in claim 2 further characterized in that said members are formed primarily of aluminum and wherein the curvature of the lapped surfaces of said members has a radius in the vicinity of 16 inches.

References Cited in the file of this patent UNITED STATES PATENTS 2,188,445 Saxe Ian. 30, 1940 2,387,134 Fox Oct. 16, 1945 2,415,573 Adams et a1 Feb. 11, 1947 2,679,305 Gunthorp May 25, 1954 2,844,864 Schilberg July 29, 1958 FOREIGN PATENTS 467,939 Canada 2 Sept. 5, 1950 

